1. Field of the Invention
The present invention relates to a zoom lens system.
2. Description of the Related Art
The market has desired a zoom lens for use in a photographic optical system of an image pickup apparatus having a high zoom ratio and a high optical performance in the entire zoom range.
In particular, in recent years, the market has desired a method for recording a still image with a high image quality with a video camera that primarily captures a moving image. Under these circumstances, it is desired that a zoom lens have a high optical performance.
Conventionally, a four-unit zoom lens is known which includes a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, in order from an object side to an image side.
Of such four-unit zoom lenses, a so-called rear focus type four-unit zoom lens is known that moves a second lens unit to vary magnification and moves a fourth lens unit to compensate for movement of an image plane caused by the variation of magnification and to perform focusing (see Japanese Patent Application Laid-Open No. 08-304700, Japanese Patent Application Laid-Open No. 2000-121941, and Japanese Patent Application Laid-Open No. 2003-295053, which corresponds to U.S. Pat. No. 6,751,029).
A rear focus type four-unit zoom lens performs focusing by moving a lens unit that is relatively small-sized and relatively light in weight. Accordingly, such a rear focus type four-unit zoom lens requires only a small drive force for driving the lens unit. This enables a quick and rapid focusing.
Japanese Patent Application Laid-Open No. 05-060971, which corresponds to U.S. Pat. No. 5,638,216, and Japanese Patent Application Laid-Open No. 2005-242014, which corresponds to U.S. Pat. No. 6,972,909, each discuss a rear focus type four-unit zoom lens having a high zoom ratio while having a high optical performance in the entire zoom range.
In the above-described rear focus type four-unit zoom lens, a configuration of a second lens unit, which functions to vary magnification, is significant to achieve a high optical performance while securing a high zoom ratio.
In rear focus type four-unit zoom lenses discussed in Japanese Patent Application Laid-Open No. 08-304700, Japanese Patent Application Laid-Open No. 2000-121941, and Japanese Patent Application Laid-Open No. 2003-295053, the second lens unit includes a lens having a negative refractive power in which an absolute value of a refractive power of a surface on the object side is larger than that on the image side, and a cemented lens including a bi-concave lens having a negative refractive and a lens having a positive refractive power, in order from the object side to the image side.
With respect to the zoom lens having such a configuration, it is difficult to correct aberration on a cemented surface of the cemented lens when the second lens unit has an increased refractive power. Accordingly, it is difficult to achieve a high optical performance in the entire zoom range.
In rear focus type four-unit zoom lenses discussed in Japanese Patent Application Laid-Open No. 05-060971 and Japanese Patent Application Laid-Open No. 2005-242014, the second lens unit includes three single lenses and does not include a cemented lens, thus achieving a high optical performance in the entire zoom range.
Particularly, in a rear focus type four-unit zoom lens discussed in Japanese Patent Application Laid-Open No. 2005-242014, a positive lens included in the second lens is made of a material having an Abbe number smaller than “20” and a refractive index higher than “1.9”. Accordingly, the refractive power of the second lens unit can be intensified to a level at which the zoom ratio as high as “20” can be secured. Accordingly, a high optical performance can be achieved in the entire zoom range.
It is useful in achieving a further higher zoom ratio to provide an appropriate principal point distance between the first lens unit and the second lens unit. In this regard, the rear principal point position can be brought closer to the object side or to bring the front principal point position closer to the image side.
In the rear focus type four-unit zoom lenses discussed in Japanese Patent Application Laid-Open No. 05-060971 and Japanese Patent Application Laid-Open No. 2005-242014, air is provided between the second and third lenses included in the second lens unit. Thus, a high zoom ratio can be achieved, while achieving a high optical performance in the entire zoom range. Furthermore, the diameter of a front lens element can be made small. If the lens configuration of the first lens unit is appropriately set, it becomes easier to achieve a high zoom ratio because the rear principal point position can be further brought closer to the object side.
Particularly, if a shape factor of two positive lenses included in the first lens unit is appropriately set, it becomes further easier to bring the rear principal point position closer to the object side, which is useful in achieving a high zoom ratio while achieving a high optical performance in the entire zoom range.